Soldering is widely used to join microelectronic assemblies. For example, solder is widely used to join electrical leads on components such as integrated circuits ("chips") to other components such as substrates including printed circuit boards, flat panel displays or the like. As is well known to those having skill in the art, solder is a low melting point alloy, usually of the lead-tin type, which is used to join metals at temperatures of about 350.degree. C. or less.
Presently, most soldering processes require precleaning and deoxidation of surface oxides prior to solder reflow, and also require post-soldering cleaning after solder reflow. The deoxidation step is generally performed with flux materials to prepare the surfaces for soldering by removing contaminants and metal oxide from the solder surface and/or the surfaces to be joined. After soldering, flux residue remaining from the deoxidation is removed during post-soldering cleaning.
The use of flux is generally undesirable because the flux residue can degrade the components or other materials which come in contact with the flux. Cleaning of the flux can also degrade the components. Moreover, as the circuit density of microelectronic devices has increased, the leads on these devices have become smaller and more closely spaced. Thus, flux has become more difficult to remove after soldering. Finally, with increasing environmental consciousness, the use of CFC-containing cleaning solvents is generally disfavored. Replacement cleaning solvents may add to the cost of the product and/or process.
In a pioneering invention, Dishon and Bobbio described a fluxless soldering process. In this process, fluorine-containing plasma excitation is performed on the solder, so that soldering may be performed without the need for flux and post-soldering cleaning. This pioneering invention is described in U.S. Pat. No. 4,921,157 to Dishon et al., issued May 1, 1990, entitled "Fluxless Soldering Process" and assigned to the assignee of the present invention, the disclosure of which is hereby incorporated herein by reference. Since the time of the Dishon et al. patent, this fluxless soldering process, now commonly referred to as the Plasma Assisted Dry Soldering (PADS) process, has become widely reported and widely used in industry.
See, for example, the publications entitled "Effect Of Water Vapor and Oxygen on Fluxless Soldering in Air and Nitrogen" by co-inventor Nangalia et al., First International Flip Chip Symposium, San Jose, Calif., February, 1994;"Fluxless, No Clean Solder Processing of Components, Printed Wiring Boards, and Packages in Air and Nitrogen" by co-inventor Koopman et al., Surface Movement International, San Jose, Calif., Aug. 29-31, 1994; "Fluxless No-Clean Assembly of Solder Bumped Flip Chips" by coinventor Koopman et al., Electronic Components and Technology Conference, Orlando, Fla., May 28-31, 1996; and "Solder Flip Chip Developments at MCNC", by coinventor Koopman et al., 1996 International Flip Chip, Ball Grid Array, Tab & Advanced Packaging Symposium, Sunnyvale, Calif., Feb. 14-16, 1996, the disclosures of which are hereby incorporated herein by reference.
The Dishon et al. patent exposed the solder to a fluorine-containing plasma. Later, fluxless soldering of copper was performed by exposing a copper surface to be soldered to a fluorine-containing plasma. See U.S. Pat. No. 5,407,121 to coinventor Koopman et al. entitled "Fluxless Soldering of Copper", assigned to the assignee of the present invention. An improved fluxless soldering system is also described in the aforesaid U.S. Pat. No. 5,407,121 and is claimed in U.S. Pat. No. 5,499,754 to Bobbio et al. entitled "Fluxless Soldering Sample Pretreating System", assigned to the assignee of the present invention. As shown, a sample chamber extension extends outwardly from an opening in a sample chamber. A fluorine-containing gas is supplied into the extension. An energy source surrounding the extension produces microwave energy in the extension to produce a plasma. Atomic fluorine traverses from the extension into the chamber so that the sample is exposed to atomic fluorine from the fluorine-containing plasma, downstream of the plasma.
The PADS process has provided breakthrough soldering technology which can eliminate flux and flux cleaning. Nonetheless, the use of flux also had a beneficial aspect with respect to tacking. When flux was used in soldering the flux acted like a glue to "tack" the chip in position during handling and subsequent reflow or soldering. Having no flux to provide the tacking, other sources of tacking may be required to successfully complete a soldering process.
Tacking is also generally required in surface mount technology, because this technology mounts component leads on the surface of a printed circuit board. Accordingly, viscous or tacky fluxes have been used to adhere the component in place prior to soldering. Unfortunately, the component adhesion requirement may place additional requirements on the flux formulation.
Thermocompression has been utilized as an alternate source of tacking. However, this process of tacking generally requires thermocompression times of 30 to 60 seconds. In a manufacturing environment, the tacking times required by thermocompression may reduce the throughput excessively.